What are 'closures' in .NET?

What are 'closures' in .NET?

I have an article on this very topic. (It has lots of examples.)

In essence, a closure is a block of code which can be executed at a later time, but which maintains the environment in which it was first created - i.e. it can still use the local variables etc of the method which created it, even after that method has finished executing.

The general feature of closures is implemented in C# by anonymous methods and lambda expressions.

Here's an example using an anonymous method:

using System;

class Test
{
    static void Main()
    {
        Action action = CreateAction();
        action();
        action();
    }

    static Action CreateAction()
    {
        int counter = 0;
        return delegate
        {
            // Yes, it could be done in one statement; 
            // but it is clearer like this.
            counter++;
            Console.WriteLine("counter={0}", counter);
        };
    }
}

Output:

counter=1
counter=2

Here we can see that the action returned by CreateAction still has access to the counter variable, and can indeed increment it, even though CreateAction itself has finished.

The Beauty of Closures